Attention deficit hyperactivity disorder and oppositional behaviours frequently co-occur, We aimed to study the etiology of this overlap in a general population-based twin sample, assessing the symptom domains of hyperactivity-impulsivity and inattentiveness separately for their overlap with oppositionality. We further aimed to investigate whether rater bias may contribute to the overlap in previous data which used one rater only. Using parent and teacher ratings on hyperactivity-impulsivity, inattentiveness and oppositionality, and actigraph measurements of activity level, for 668 7-9-year-old twin pairs, oppositionality showed a higher overlap with hyperactivity-impulsivity (r = 0.95) than with inattentiveness (r = 0.52) and all etiological influences on hyperactivity-impulsivity were shared with those on oppositionality, indicated by a genetic correlation of 0.95 and a child-specific environmental correlation of 0.94. Actigraph data did not show an overlap with ratings of oppositionality. In middle childhood, symptoms of hyperactivity-impulsivity and oppositional behaviour may represent the same underlying liability, whereas the inattentive domain is more distinct.